a) Field of the Invention
The invention is directed to a process for producing composite bodies from a plurality of molded pieces made from quartz materials of different or identical structure, preferably fused quartz and/or fused silica. The invention is further directed to composite bodies produced according to this process.
b) Description of the Related Art
The generic type of composite bodies of quartz material to which the invention also belongs is used primarily as supporting or carrying plates for intermediate products in semiconductor fabrication, for example, for silicon disks and wafers, and also in high-temperature processes which take place, above all, under cleanroom conditions. They are also conventionally used in technological processes in the optics and chemicals industries.
The composite bodies which are usually constructed so as to be plate-shaped are generally formed from a large-area rectangular fused-quartz plate which is provided with an edge of fused silica. The thickness of a composite body used as a support of this type ranges from 4 mm to 10 mm. The fused quartz plate provides the actual supporting surface, while the edge has the function of reducing the flow of heat from the object, via the fused quartz plates, toward the edge areas during thermal processing or machining steps directed to the supported object and to keep the heat in the support area as far as possible.
In the production of plate-shaped composite bodies of this type, the special nature of the physical and chemical properties of the quartz material must be taken into account, with the result that special technologies had to be developed. Accordingly, it is known, for example, to produce composite bodies of the type mentioned above separately from a fused quartz plate cut in rectangular shape and four fused silica edge pieces, the fused silica edge pieces generally being connected with the fused quartz plate by welding with a hydrogen flame. It is also known to connect by gluing instead of by welding.
It is disadvantageous that this production process is extremely cost-intensive because the individual fabrication of the fused quartz plates and fused silica edge pieces, as well as their connection with one another, is very time-consuming. Further, there is a very large amount of wasted quartz material. Increasing production piece numbers, for instance, in the semiconductor industry, also require the development of production methods by which the described composite bodies can be produced much more efficiently.
Moreover, individual fabrication is also disadvantageous due to the limited repeating accuracy from one composite body to the next with respect to dimensional stability as well as accuracy of geometric shape.
Further, cracks also frequently form in the material due to the mechanical stresses resulting from varying expansion of the parts that are joined to one another or because of inhomogeneous material expansion due to locally limited application of the welding flame.
Insofar as gluing connections are selected over welding connections, the composite bodies produced in this way cannot be used in the high-temperature range because the applicable glue connections can only withstand temperatures of up to about 300xc2x0 C. and, further, the glued locations also exhale gas when heated, which is undesirable in many technical processes.
The production of one-piece fused quartz plates is known from DE-OS 32 26 451, in which plates are sawed from a fused quartz cylinder and subsequently cut to the desired shape, e.g., a rectangle. However, this publication makes no reference to the production of plate-shaped composite bodies from a plurality of quartz materials.
It is also already known to produce tube-shaped and/or flange-shaped composite bodies from fused silica and fused quartz, as is described, for example, in DE 29 08 288, DE 31 41 919 and DE 44 40 104. However, the processes indicated in these references are not applicable to the production of plate-shaped composite bodies suitable as supports for the purposes mentioned above.
Proceeding from the disadvantages in the prior art mentioned above, it is the object of the invention to provide a process of the type mentioned in the beginning which enables an economical series manufacture with high material yields and high repeating accuracy and in which cracking is prevented to a great extent at the same time.
According to the invention, this object is met in that a prismatic molded piece with a rectangular bottom surface and top surface and four plate-shaped molded pieces whose dimensions are tailored to the outer surfaces of the prismatic molded piece are produced in a first step, the plate-shaped molded pieces are then fixed to the outer surfaces of the prismatic molded piece, the prismatic molded piece and the plate-shaped molded pieces are heated together under protective gas to a predetermined temperature Txcex5 above the softening temperature of the quartz material and then cooled, resulting in a prismatic composite body which is joined thermally in a material engagement.
In a preferred arrangement of the invention, the plate-shaped molded pieces are fixed to the outer surfaces of the prismatic molded piece using a graphite mold. Further, it has proven advantageous when the temperature Txcex5 is maintained for a predetermined time period txcex5.
The prismatic composite body formed in this way can be cut repeatedly parallel to its bottom surface or top surface for the purpose of producing a series of plate-shaped composite bodies, wherein the thickness of the plate-shaped composite bodies is determined in each instance by the distance from one cutting plane to the next cutting plane.
Accordingly, in the process according to the invention, a larger prismatic or block-shaped composite body which is joined thermally in material engagement is first produced from quartz materials of different or identical structure, but with expansion coefficients which are always at least approximately identical, and a series of plate-shaped composite bodies with identically shaped contours are then obtained therefrom.
In this way, a locally limited heating during production is advantageously avoided and composite bodies are obtained which are free from stresses or low in stresses. The risk of cracking and its deleterious consequences can virtually be ruled out. The joint zone is narrowly defined geometrically and is not surrounded by a mixed zone of materials of the adjoining molded pieces.
Accordingly, application of the process according to the invention enables an economical series manufacture of plate-shaped composite bodies with a high material yield and relatively little waste.
In a particularly preferred arrangement of the process according to the invention, the fixing of the molded pieces is carried out using a graphite mold. Further, it is provided that the plate-shaped molded pieces are fixed to the outer surfaces of the prismatic molded piece in such a way that the connection surfaces of every two adjoining molded pieces facing each other are inclined relative to one another at an angle xcex1, wherein the molded pieces are oriented relative to one another in such a way that the distance between these surfaces is greater in the direction opposite to the direction of gravitational force. This has the advantageous result that the gases inevitably occurring during the heating and holding periods can escape upward from the joint gap unimpeded, while the connection of the respective surfaces and molded pieces proceeds from the bottom to the top opposite the direction of gravitational force; in particular, joint zones which are free from bubbles or have few bubbles are achieved in this way.
It has proven particularly advantageous to heat the molded pieces held in the graphite mold at a heating rate of 5 to 10xc2x0 C. per minute to a temperature Txcex5 of 1710xc2x0 C. to 1790xc2x0 C. The holding time txcex5 should advisably be 3 to 8 hours.
The cooling of the thermally joined block-shaped composite body is advantageously carried out in a defined manner at a cooling rate of 0.1 to 1xc2x0 C. per minute.
The individual plate-shaped composite bodies can be cut from the block-shaped composite body parallel to the bottom surface by means of different methods; however, sawing by means of a diamond saw has proven advisable.
The process is also advantageously applicable for the production of correspondingly joined bodies from molded pieces made from materials that are materially related to quartz materials.
The object is further met according to the invention by a plate-shaped composite body which is produced by means of the process described above and which is formed of a plate with a rim or edge which extends around the entire circumference and is joined without intermediate layers in a material engagement.
In this respect, it is a substantial advantage that the joint zones of this composite body are narrowly defined and not surrounded by a mixed zone of different materials of adjacent molded pieces. They are free from bubbles or have few bubbles and, as a result of the process according to the invention, the composite body has only negligible stresses, if any, so that cracks are not expected to occur in the material.
In a particularly advantageous arrangement, the large-area plate is made from fused quartz and the edge joined thereto is made from fused silica. In this way, especially when used as a support in high-temperature processes, a good retention of heat can be achieved in the plate or in the product placed on the fused quartz plate because the more porous edge made from fused silica has a lower heat conductivity compared with the fused quartz of the plate and, therefore, the heat cannot flow hastily into the edge.
In a further arrangement, the surface of the composite body comprising the plate and edge is ground and polished. The circumferential outer surface of the composite body is advisably rounded and polished or broken into facets and polished.
The invention will be described more fully in the following with reference to an embodiment example.